DIY Guitar amplifier keeps blowing caps...No clue what to try

Discussion in 'The Projects Forum' started by mrwest09, Jun 26, 2013.

  1. mrwest09

    Thread Starter New Member

    Mar 23, 2013
    7
    0
    Hey everyone,

    I'm working on building a DIY guitar amplifier kit from Trinity Amps (the Tweed model). I have the entire thing soldered but when I turned it on one of the capacitors (C9 in the attached diagram) started to fail (a bubble formed on the outside). Not only that but even the indicator light shown at the bottom of the diagram would turn on and when I touched the case I got a little buzz.

    I am stuck as I am not sure what I can test without turning the amplifier on as every time I do the cap is wrecked and I have to wait to order a new one. I feel like something is wrong with the power transformer but I'm not sure at all.

    I've attached three pictures of the same schematic. Hopefully someone can point me in the right direction!!
    [​IMG]
    [​IMG]
    [​IMG]
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    951
    Verify the connections on your power tube sockets. Look closely and be sure that you have them correct. I suspect you may have connected wires on the wrong pins. Be sure to account for the difference in numbering direction when viewing from top or bottom of the tube socket.
     
  3. #12

    Expert

    Nov 30, 2010
    16,341
    6,824
    The capacitor is not connected until you turn on the B+ switch. Leave it off, then put a 600V film cap on the B+ and see if it's positive. .33uf, .47uf, doesn't matter because there is no load with the B+ switch turned off. Then measure the voltage. If you installed silicon rectifiers, the B+ will be too high. Check the power line voltage. I get 125 volts at my house, and that will raise your output to 516 volts.

    Then tell what country you are in so we can guess what power you are supposed to be getting from the wall outlet.
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Then, after checking the B+ you can remove C9 (and C8) also. With those removed, check the voltages on the tube sockets.
     
  5. mrwest09

    Thread Starter New Member

    Mar 23, 2013
    7
    0
    Thanks for all your help guys! I will try this out and get back to you!

    Oh and when adding the cap to the B+ does it have to be a film cap? I have a number of different types kicking around my place but I'm not sure if I have one of them. Also, I'm assuming I connect this cap from the B+ connection to ground?

    Sorry for these basic questions... I'm new to this sort of circuit design business :)
     
    Last edited: Jun 27, 2013
  6. #12

    Expert

    Nov 30, 2010
    16,341
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    The film capacitor goes from the rectifier to ground. It must be poly-something film so it will be non-polarized and survive over 500 volts. You already HAVE some. Try C5 or C6 and see if they are rated at a high enough voltage. Any other capacitor has the possibility of exploding.
     
  7. mrwest09

    Thread Starter New Member

    Mar 23, 2013
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    0
    I had a chance to test the circuit today. B+ was found to be 479V, on the 5V coil we are getting closer to 4.7V, on connection 4 at the tube we get 330V, on connection 6 at the tube was also 330V and the input voltage was 120.7V. The indicator light was reading no voltage at all. Not only that, but we noticed the transformer was getting quite hot when the switch for the entire system was off. Clearly there is something wrong with the power portion of our design but we still aren't sure what. Do you think we may have a bad transformer?
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    You might have a short across the filament winding if you have a hot transformer and no pilot lamp. Check the filament connections. If you see no problems with the wiring, disconnect the 6.3 v filament leads and check the voltage there.
     
  9. #12

    Expert

    Nov 30, 2010
    16,341
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    Look for a mechanical connection to chassis at the indicator light. That would be a mistake that is easy to make.
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    Have you got the input connections on the transformer primary in the correct phasing as per the diagram colours, that maybe giving your transformer the excess heat?
     
  11. Jon Wilder

    New Member

    Oct 25, 2011
    23
    1
    If C9 failed, I'm gonna venture to guess that either R10 or R11 in the cathode circuit of the phase inverter side of V2 is open and/or not grounded. This would cause the cathode to be pulled up to a voltage close to the rail, which could send that cap over voltage. Check the voltage on pin 8 of V2. If it's far above 50V, I'd start there.

    As to the indicator lamp issue, measure the voltage across it. Loaded it should be close to 6.3VAC. If it's vastly lower than that, look for a short on one of the heater connections on the other tubes.

    A few photos of the amp and its internal wiring just might help here as well.
     
  12. mrwest09

    Thread Starter New Member

    Mar 23, 2013
    7
    0
    Alright guys, I finally had a chance to look at the amp again and things are not well. I looked at EVERY part of this stupid indicator light and cannot see a mechanical connection to ground. I disconnected the bulb and and was able to light it up with a 9V battery so the bulb is definitely still good. Not only that but with the bulb disconnected I: a) blew the 2A fuse somehow and b) shocked myself again. This is starting to feel more and more like a bad movie. Any other suggestions? I basically just don't want to blow the fuse again (I'm not sure how that happened in the first place) as it really will set me back time-wise.
     
  13. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Are you sure that the C9 you are using is an unpolarized cap rated at 200V?
     
  14. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
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    mrwest09
    What voltage did you get on the filament winding. ?
     
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